1. Field of the Invention
The present invention relates to stabilizing and supporting a focal plane array (FPA) on a cooling platform. More particularly, the present invention provides for enhanced reliability of the FPA throughout thermal cycling processes during which temperature-induced contraction and expansion between dissimilar materials comprising the FPA may significantly vary.
2. Description of Related Art
A further discussion of FPA mounting and stiffening techniques is described in copending application entitled "Hybrid Focal Plane Array Stabilizing Apparatus And Method," Serial No. 08/409,230, filed Mar. 23, 1995, and invented by R. K. Asatourian, D. E. Cooper, W. L. Morris, and M. R. James. The disclosure of the aforementioned copending application is incorporated herein by reference.
Hybrid FPAs, by definition, are formed of a variety of different materials having differing coefficients of thermal expansion. Accordingly, the conventional layering scheme of such FPAs often causes the materials incorporated in the FPA to be distorted and deflected as the result of the potentially wide changes in temperatures applied to the FPA.
As illustrated in FIG. 1, FPAs typically include several different layers of materials, such as an optical substrate layer 100 coated with an optically-sensitive material 102, an interconnecting scheme typically formed of pliable conducting bumps 104, and an electronic means 106, such as a multiplexer (MUX) layer which includes the FPA electronics. The above-described materials of the FPA are mounted on a platform or base 110 in a layered arrangement over a full-face epoxy layer 108. The system is cooled by cooling the mounting platform which, in turn, cools the FPA. Infrared radiation enters the optical substrate layer and is detected by the layer of optically-sensitive material.
However, like any system of dissimilar materials exposed to thermal excursions, the FPA and its mounting configuration must meet predetermined thermal cycling reliability objectives of a particular application. If temperature variations are sufficiently great and the thermal expansion coefficients (TECs) of the materials used in the FPA are substantially different, damage to the interconnection scheme between each of the layers of the FPA and the optically sensitive material can occur after a number of cycles.
For example, the hybrid interface of the FPA, e.g., the interconnecting bumps between the detector layer and the MUX, is pliable and thermal contraction can damage these interconnections which form the electrical and mechanical contacts for each pixel of the FPA. Large strains on the interconnect bumps may break the connections, opening the electrical contact between the two layers. Stresses in this region can also lead to damage of the optically sensitive detector layer, reducing its photo-response and increasing the noise.
More particularly, with regard to lateral, or in-plane, deformations of the interconnect bumps, it has been found that such deformations are typically caused by the difference in the contraction rate of the hybrid FPA components. In addition to in-plane deformation, such contraction rate differences can induce out-of-plane forces on the FPA leading to bowing of the FPA. For example, in a typical hybrid FPA in which the detector material has a TEC higher than the MUX, the hybrid FPA will assume a concave deflection or bow when cooled.
Furthermore, this undesirable bowing may be affected by the mounting of the hybrid FPA on a supporting platform, such as the cold plate of a dewar. The deformations experienced by the mounting platform may then be transmitted to the FPA, and may increase or decrease the FPA bow depending on the direction of deflection of the platform. More particularly, the distortions and deflections experienced by the FPA may potentially be further increased by deflections and distortions that the platform itself experiences when it is cooled down. Thus, due to the mismatch of TECs of various layers in the FPA and the platform, as well as the deflections of the platform itself, the interconnect bumps may experience both an in-plane and an out-of-plane deformation leading to their work-hardening, and eventual fracture or separation.
One approach used to reduce damage to the interconnect bumps has been to back-fill the space between the MUX and the detector with epoxy. This epoxy takes some of the mechanical load off the bumps, and helps to hold the MUX and the detector layers together, thereby reducing bump damage. However, the back-fill epoxy, which generally has a TEC that is higher than both the MUX and the optical substrate forming the detector, also has a tendency to produce lateral tension on the bumps. Since the epoxy adheres to the surface of the optically sensitive material, it tends to strain the layer and degrade its characteristics.
In another approach, the MUX is forced to comply with the TEC of the detector optical substrate by mounting a thin MUX on a rigid material which has a TEC which matches that of the detector substrate. Although this approach tends to be less stressful on the optically sensitive material, certain practical challenges are introduced when the MUX circuitry and interconnect bumps are processed on a thin layer, or when the MUX layer is thinned after the electronic circuitry and bumps have been fabricated. In addition, functionality problems have been observed in thin MUXs where the electronic circuitry is subjected to extremely high stress levels both from the thinning process and from the compressive force during cooling. Neither of these approaches are immune to the impact of platform deflections.